WO2024203488A1

WO2024203488A1 - Gel material having four-branched crosslinking points at which strain-induced crystallization is developed

Info

Publication number: WO2024203488A1
Application number: PCT/JP2024/010379
Authority: WO
Inventors: 皓一眞弓; 崇匡酒井; シャンリ; 慧橋本
Original assignee: 国立大学法人東京大学
Priority date: 2023-03-29
Filing date: 2024-03-15
Publication date: 2024-10-03

Abstract

Provided is a gel composition comprising: a polymer network in which a first polymer that is a four-branched or linear polymer having an electrophilic functional group and a polyethylene glycol framework and a second polymer that is a four-branched or linear polymer having a nucleophilic functional group and a polyethylene glycol framework are crosslinked with each other, wherein the first polymer and/or the second polymer is a four-branched polymer; and a solvent. In the gel composition, the total amount of the first polymer and the second polymer per the amount of the gel composition is 30% by weight or more, and the molecular weight of a moiety between crosslinking points of the polymer networks is 10000 or more.

Description

Gel material with four-branch crosslinks exhibiting elongation-induced crystallization

The present invention relates to a gel material containing a polymer network formed by crosslinking four-branched polymers.

Materials in which a solvent is trapped in a crosslinked polymer network are called polymer gels, and hydrogels using water as a solvent are expected to be used as biomedical materials such as artificial joints, while ion gels using ionic liquid as a solvent are expected to be used as electrolytes in wearable devices. To realize these applications, it is necessary to improve the mechanical toughness of polymer gels. In recent years, the use of stretch-induced crystallization, in which oriented polymers crystallize during stretching to prevent breakage, has attracted attention as a method for toughening polymer gels. In a cyclic hydrogel in which polyethylene glycol (PEG) chains are crosslinked with cyclic molecules, it was reported in 2021 that stretch-induced crystallization occurs when the PEG chains are uniformly oriented due to the sliding freedom of the cyclic molecules, resulting in significant toughening (Non-Patent Document 1). We have also found that stretch-induced crystallization occurs in cyclic ion gels in which the solvent is ionic liquid or salt, and have applied for a patent (Patent Document 1). In addition, in 2022, it was reported that in a PEG hydrogel with three crosslinking branching points, a crosslinked network consisting only of polymer chains without using cyclic molecules, stretch-induced crystallization occurred, leading to excellent extensibility (Non-Patent Document 2). It is known that crosslinking branching points inhibit the crystallization of polymer chains because polymer chains cannot approach each other. In the case of the three-branched hydrogel, the smallest number of branching points is thought to have led to the efficient occurrence of stretch-induced crystallization.

On the other hand, the development of 4-branch cross-linked PEG hydrogels and ionic gels has been reported in previous papers and patents, but stretch-induced crystallization was not observed in either, and the breaking stress during uniaxial stretch testing was less than 1 MPa (Non-patent literature 3, 4).

Patent Application No. 2022-113741 (National University Corporation, University of Tokyo) 07.15.2022

The problem to be solved by this invention is to provide a gel composition with four branched crosslinking points that exhibits stretch-induced crystallization.

The present invention encompasses the embodiments described below.
Item 1.
1. A gel composition comprising:
A polymer network formed by crosslinking a first polymer, which is a 4-branched or linear polymer having an electrophilic functional group and a polyethylene glycol backbone, with a second polymer, which is a 4-branched or linear polymer having a nucleophilic functional group and a polyethylene glycol backbone, wherein one or both of the first polymer and the second polymer are 4-branched polymers;
A solvent,
A gel composition, wherein the total amount of the first polymer and the second polymer relative to the amount of the gel composition is 30% by weight or more, and the molecular weight between crosslinking points of the polymer network is 10,000 or more.

Item 2.
Item 2. The gel composition according to item 1, wherein the electrophilic functional group of the first polymer is an active ester group, a maleimidyl group, a carboxyl group, an N-hydroxy-succinimidyl group, a sulfosuccinimidyl group, a phthalimidyl group, or a nitrophenyl group.

Item 3.
Item 2. The gel composition according to item 1, wherein the nucleophilic functional group of the second polymer is an amino group, a thiol group, or -CO ₂ PhNO ₂ (Ph is an o-, m-, or p-phenylene group).

Item 4.
Item 2. The gel composition according to item 1, wherein the solvent comprises at least one selected from the group consisting of water, an organic solvent, and an ionic liquid.

Item 5.
Item 2. The gel composition according to item 1, wherein crystals of the polyethylene glycol skeleton are formed by uniaxial stretching.

Item 6.
Item 2. The gel composition according to Item 1, wherein a gel sheet made of the gel composition according to Item 1 is punched into a dumbbell shape having a thickness of 1 mm according to JIS K 6251 No. 3, and the gel sheet is deformed by uniaxial stretching at an elongation rate of 12.5%/sec, and the breaking stress is measured to be 1 MPa or more.

Item 7.
The first polymer is a compound represented by the following formula (I) or a compound represented by the following formula (II):

(In formula (I), n ₁ to n ₄ may be the same or different, and n ₁ to n ₄ are integers of 5 to 300.
A is a maleimidyl group, an N-hydroxy-succinimidyl group, a sulfosuccinimidyl group, a phthalimidyl group, an imidazoyl group, an acryloyl group or a nitrophenyl group;
R ¹ to R ⁴ may be the same or different and each represents an alkylene group having 1 to 7 carbon atoms, an alkenylene group having 2 to 7 carbon atoms, -NH-R ⁵ -, -CO-R ⁵ -, -R ⁶ -OR ⁷ -, -R ⁶ -NH-R ⁷ -, -R ^6- COO-R ⁷ -, -R ⁶ -COO-NH-R ⁷ -, -R ⁶ -CO-R ⁷ -, -R ⁶ -NH-CO-R ⁷ -, or -R ⁶ -CO-NH-R ⁷ -, where R ⁵ represents an alkylene group having 1 to 7 carbon atoms, R ⁶ represents an alkylene group having 1 to 3 carbon atoms, and R ⁷ represents an alkylene group having 1 to 5 carbon atoms.

(In formula (II), n ₁₂ is an integer of 1 to 10,000;
B is a maleimidyl group, an N-hydroxy-succinimidyl group, a sulfosuccinimidyl group, a phthalimidyl group, an imidazoyl group, an acryloyl group, or a nitrophenyl group;
R ¹¹ represents an alkylene group having 1 to 7 carbon atoms, an alkenylene group having 2 to 7 carbon atoms, -NH-R ¹³ -, -CO-R ¹³ -, -R ¹⁴ -OR ¹⁵ -, -R ¹⁴ -NH-R ¹⁵ -, -R ¹⁴ -COO-R ¹⁵ -, -R ¹⁴ -COO-NH-R ¹⁵ -, -R ¹⁴ -CO-R ¹⁵ -, R ¹⁴ -NH-CO-R ¹⁵ - or -R ¹⁴ -CO-NH-R ¹⁵ -, wherein R ¹³ represents an alkylene group having 1 to 7 carbon atoms, R ¹⁴ represents an alkylene group having 1 to 3 carbon atoms, and R ¹⁵ represents an alkylene group having 1 to 5 carbon atoms;
R ¹² represents an alkylene group having 1 to 7 carbon atoms, an alkenylene group having 2 to 7 carbon atoms, -NH-R ¹⁶ -, -CO-R ¹⁶ -, -R ¹⁷ -OR ¹⁸ -, -R ¹⁷ -NH-R ¹⁸ -, -R ¹⁷ -COO-R ¹⁸ -, -R ¹⁷ -COO-NH-R ¹⁸ -, -R ¹⁷ -CO-R ¹⁸ -, R ¹⁷ -NH-CO-R ¹⁸ - or -R ¹⁷ -CO-NH-R ¹⁸ -, where R ¹⁶ represents an alkylene group having 1 to 7 carbon atoms, R ¹⁷ represents an alkylene group having 1 to 3 carbon atoms, and R ¹⁸ represents an alkylene group having 1 to 5 carbon atoms.
Item 2. The gel composition according to item 1, wherein the second polymer is a compound represented by the following formula (III) or a compound represented by formula (IV):

(In formula (III), n ₂₁ to n ₂₄ may be the same or different, and n ₂₁ to n ₂₄ are integers of 25 to 250.
D is a thiol group, an amino group, or _-COOPhNO2 (Ph represents an o-, m-, or p-phenylene group);
R ²¹ to R ²⁴ may be the same or different and each represents an alkylene group having 1 to 7 carbon atoms, an alkenylene group having 2 to 7 carbon atoms, -NH-R ²⁵ -, -CO-R ²⁵ -, -R ²⁶ -OR ²⁷ -, -R ²⁶ -NH-R ²⁷ -, -R ²⁶ -COO-R ²⁷ -, -R ²⁶ -COO-NH-R ²⁷ -, -R ²⁶ -CO-R ²⁷ -, R ²⁶ -NH-CO-R ²⁷ - or -R ²⁶ -CO-NH-R ²⁷ -, where R ²⁵ represents an alkylene group having 1 to 7 carbon atoms, R ²⁶ represents an alkylene group having 1 to 3 carbon atoms, and R ²⁷ represents an alkylene group having 1 to 5 carbon atoms.

(In formula (IV), n ₃₂ is an integer from 1 to 10,000;
E is a thiol group, an amino group, or _-COOPhNO2 (Ph represents an o-, m-, or p-phenylene group);
R ³¹ represents an alkylene group having 1 to 7 carbon atoms, an alkenylene group having 2 to 7 carbon atoms, -NH-R ³³ -, -CO-R ³³ -, -R ³⁴ -OR ³⁵ -, -R ³⁴ -NH-R ³⁵ -, -R ³⁴ -COO-R ³⁵ -, -R ³⁴ -COO-NH-R ³⁵ -, -R ³⁴ -CO-R ³⁵ -, R ³⁴ -NH-CO-R ³⁵ - or -R ³⁴ -CO-NH-R ³⁵ -, where R ³³ represents an alkylene group having 1 to 7 carbon atoms, R ³⁴ represents an alkylene group having 1 to 3 carbon atoms, and R ³⁵ represents an alkylene group having 1 to 5 carbon atoms;
R ³² represents an alkylene group having 1 to 7 carbon atoms, an alkenylene group having 2 to 7 carbon atoms, -NH-R ³⁶ -, -CO-R ³⁶ -, -R ³⁷ -OR ³⁸ -, -R ³⁷ -NH-R ³⁸ -, -R ³⁷ -COO-R ³⁸ -, -R ³⁷ -COO-NH-R ³⁸ -, -R ³⁷ -CO-R ³⁸ -, R ³⁷ -NH-CO-R ³⁸ - or -R ³⁷ -CO-NH-R ³⁸ -, where R ³⁶ represents an alkylene group having 1 to 7 carbon atoms, R ³⁷ represents an alkylene group having 1 to 3 carbon atoms, and R ³⁸ represents an alkylene group having 1 to 5 carbon atoms.

Item 8.
Item 8. A gel sheet, which is a sheet formed from the gel composition according to any one of Items 1 to 7.

Overview of gel compositions of the present embodiments. (A) Stress-extension curves for three types of 4-branched hydrogels with different molecular weights between crosslinks (all PEG concentrations are 40 wt%). (B)-(D) Wide-angle X-ray scattering (WAXS) images at maximum extension ratio: (B) molecular weight between crosslinks of 5k (Tetra 10k + Tetra 10k), (C) molecular weight between crosslinks of 10k (Tetra 20k + Tetra 20k), (D) molecular weight between crosslinks of 30k (Linear 20k + Tetra 20k). Elongation of the WAXS image in Figure 2(C) and the vertical WAXS one-dimensional profile (λ = 10, 11). The elongation of the WAXS image in Figure 2(D) and the vertical WAXS one-dimensional profile. (A) Stress-elongation ratio curves for three types of 4-branched hydrogels with different PEG concentrations (Linear 20k + Tetra 20k, all with molecular weight between crosslinks of 30k). (B)-(D) Wide-angle X-ray scattering at maximum elongation ratio. (B) PEG concentration 10 wt%, (C) PEG concentration 20 wt%, (D) PEG concentration 40 wt%. (A) WAXS image of a four-branched ion gel under stretching, (B) ionic liquid, and (C) one-dimensional WAXS profile perpendicular to the stretching direction. WAXS images of four-branched hydrogels obtained using four different ionic liquids: [C ₂ mim][NTf ₂ ], Li[NTf ₂ ], [Li(G3)][NTf ₂ ], and [Li(G4)][NTf ₂ ].

In this specification, the singular forms (a, an, the) include both the singular and the plural, unless otherwise expressly stated in the specification or clearly contradictory in the context.

In this specification, "comprise" is a concept that also encompasses "consist essentially of" and "consist of."

In the numerical ranges described in this specification in stages, the upper or lower limit of a certain numerical range can be arbitrarily combined with the upper or lower limit of a numerical range of another stage. Furthermore, in the numerical ranges described in this specification, the upper or lower limit of the numerical range may be replaced with a value shown in an example or a value that can be unambiguously derived from an example. Furthermore, in this specification, a numerical value connected with "~" means a numerical range that includes the numerical values before and after "~" as the upper and lower limits.

In this invention, we discovered that stretch-induced crystallization occurs in a four-branch cross-linked polymer gel, which has a more common cross-linked structure, and found that excellent toughness can be achieved.

According to one aspect of the present invention, there is provided a gel composition comprising a polymer network formed by crosslinking a first polymer, which is a four-branched or linear polymer having electrophilic functional groups and a polyethylene glycol backbone, and a second polymer, which is a four-branched or linear polymer having nucleophilic functional groups and a polyethylene glycol backbone, where one or both of the first polymer and the second polymer are four-branched polymers, and a solvent, the total amount of the first polymer and the second polymer relative to the amount of the gel composition being 30% by weight or more, and the molecular weight between crosslinking points of the polymer network being 10,000 or more.

"A four-branched polymer with a polyethylene glycol backbone" is a hydrophilic polymer with a structure in which four polyethylene glycol chains branch out from the center. A gel made of such a four-branched polyethylene glycol backbone is generally known as a Tetra-PEG gel.

A polymer network is constructed by an AB type cross-end coupling reaction between two four-branched polymers, the first polymer having electrophilic functional groups and the second polymer having nucleophilic functional groups (Matsunaga et al., Macromolecules, Vol. 42, No., pp. 1344-1361, 2009).

Tetra-PEG gel can be easily prepared on-site by simply mixing two polymer solutions, and it is also possible to control the gelation time by adjusting the pH and ionic strength during gel preparation. Furthermore, because the main component of this gel is PEG, it also has excellent biocompatibility.

When the total amount of the first polymer and the second polymer relative to the amount of the gel composition is 30% by weight or more, and the molecular weight between the crosslinking points of the polymer network is 10,000 or more, stretch-induced crystallization occurs, resulting in a toughened gel composition. When the total amount of the first polymer and the second polymer relative to the amount of the gel composition is less than 30% by weight, or when the molecular weight between the crosslinking points of the polymer network is less than 10,000, stretch-induced crystallization does not occur even when the gel composition is stretched to breakage.

The 4-branched polymer used as the first polymer preferably has one or more electrophilic functional groups at the side chain or at the end. The 4-branched polymer used as the second polymer preferably has one or more nucleophilic functional groups at the side chain or at the end.

The electrophilic functional group of the first polymer can be an active ester group. Examples of such active ester groups include maleimidyl groups, carboxyl groups, N-hydroxy-succinimidyl (NHS) groups, sulfosuccinimidyl groups, phthalimidyl groups, imidazoyl groups, acryloyl groups, and nitrophenyl groups, and those skilled in the art can use any known electrophilic functional group as appropriate. In a preferred embodiment, the electrophilic functional group of the first polymer is selected from the group consisting of active ester groups, maleimidyl groups, carboxyl groups, N-hydroxy-succinimidyl groups, sulfosuccinimidyl groups, phthalimidyl groups, and nitrophenyl groups. In a further preferred embodiment, all of the electrophilic functional groups of the first polymer are active ester groups, maleimidyl groups, carboxyl groups, N-hydroxy-succinimidyl groups, sulfosuccinimidyl groups, phthalimidyl groups, or nitrophenyl groups.

The electrophilic functional groups of the first polymer may be the same or different, but it is preferable that they are the same. By having the functional groups be the same, the reactivity with the nucleophilic functional groups that form crosslinks is uniform, and a gel with a uniform three-dimensional structure can be obtained.

The nucleophilic functional group of the second _polymer may be an amino group, a thiol group (-SH), or _-CO2PhNO2 (Ph represents an o-, m-, or p-phenylene group), and a person skilled in the art may appropriately use a known nucleophilic functional group. In a preferred embodiment, the nucleophilic functional group of the second polymer is selected from the group consisting of an amino group, a thiol group (-SH), and _CO2PhNO2 (Ph represents an o-, m-, or p-phenylene group). In _a more preferred embodiment, all of the nucleophilic functional groups of the second polymer are an amino group, a thiol group ( _-SH ), or _-CO2PhNO2 (Ph represents an o-, m-, or p-phenylene group).

The nucleophilic functional groups of the second polymer may be the same or different, but it is preferable that they are the same. By using the same functional groups, the reactivity with the electrophilic functional groups that form cross-links is uniform, making it easier to obtain a gel with a uniform three-dimensional structure.

In a preferred embodiment, the first polymer is a compound represented by the following formula (I) or a compound represented by the following formula (II):

(In formula (II), n ₁₂ is an integer of 1 to 10,000;
B is a maleimidyl group, an N-hydroxy-succinimidyl group, a sulfosuccinimidyl group, a phthalimidyl group, an imidazoyl group, an acryloyl group, or a nitrophenyl group;
R ¹¹ represents an alkylene group having 1 to 7 carbon atoms, an alkenylene group having 2 to 7 carbon atoms, -NH-R ¹³ -, -CO-R ¹³ -, -R ¹⁴ -OR ¹⁵ -, -R ¹⁴ -NH-R ¹⁵ -, -R ¹⁴ -COO-R ¹⁵ -, -R ¹⁴ -COO-NH-R ¹⁵ -, -R ¹⁴ -CO-R ¹⁵ -, R ¹⁴ -NH-CO-R ¹⁵ - or -R ¹⁴ -CO-NH-R ¹⁵ -, wherein R ¹³ represents an alkylene group having 1 to 7 carbon atoms, R ¹⁴ represents an alkylene group having 1 to 3 carbon atoms, and R ¹⁵ represents an alkylene group having 1 to 5 carbon atoms;
R ¹² represents an alkylene group having 1 to 7 carbon atoms, an alkenylene group having 2 to 7 carbon atoms, -NH-R ¹⁶ -, -CO-R ¹⁶ -, -R ¹⁷ -OR ¹⁸ -, -R ¹⁷ -NH-R ¹⁸ -, -R ¹⁷ -COO-R ¹⁸ -, -R ¹⁷ -COO-NH-R ¹⁸ -, -R ¹⁷ -CO-R ¹⁸ -, R ¹⁷ -NH-CO-R ¹⁸ - or -R ¹⁷ -CO-NH-R ¹⁸ -, where R ¹⁶ represents an alkylene group having 1 to 7 carbon atoms, R ¹⁷ represents an alkylene group having 1 to 3 carbon atoms, and R ¹⁸ represents an alkylene group having 1 to 5 carbon atoms.

In the above formula (I), n ₁ to n ₄ may be the same or different. The closer the values of n ₁ to n ₄ are, the more uniform the gel can be in a three-dimensional structure and the higher the strength, and it is preferable that they are the same. If the values of _{n 1} to n ₄ are too high, the strength of the gel will be weak, and if the values of n ₁ to n ₄ are too low, the gel will not be easily formed due to the steric hindrance of the compound. Therefore, n ₁ to n ₄ may be, for example, an integer value of 5 to 300, preferably 20 to 250, more preferably 30 to 180, and even more preferably 45 to 115. In one embodiment, n ₁ to n ₄ are integers of 5 to 300. The molecular weight of the compound represented by formula (I) is preferably 5×10 ³ to 5×10 ⁴ Da, preferably 7.5×10 ³ to 3×10 ⁴ Da, and more preferably 1×10 ⁴ to 2×10 ⁴ Da.

In the above formula (I), R ¹ to R ⁴ are linker moieties that connect the functional group and the core moiety. R ¹ to R ⁴ may be the same or different, but are preferably the same in order to produce a high-strength gel having a uniform three-dimensional structure.

In the above formula (II), _n12 can be, for example, an integer value of 1 to 10,000, preferably 25 to 5,000, more preferably 50 to 2,500, and even more preferably 100 to 1,000. The molecular weight of the compound represented by formula (II) is preferably 1.1×10 ³ to 2.2×10 ⁵ Da, preferably 2.2×10 ³ to 1.1×10 ⁵ Da, and more preferably 4.4×10 ³ to 4.4×10 ⁵ Da.

Here, examples of the alkylene group having 1 to 7 carbon atoms for each of R ¹ to R ⁴ , R ¹¹ , and R ¹² include a methylene group, an ethylene group, a propylene group, and a butylene group. The alkenylene group having 2 to 7 carbon atoms is a linear or branched alkenylene group having 2 to 7 carbon atoms and having one or more double bonds in the chain, and examples thereof include a divalent group having a double bond formed by removing 2 to 5 hydrogen atoms from adjacent carbon atoms of the alkylene group having 1 to 7 carbon atoms.

In a preferred embodiment, the second polymer is a compound represented by the following formula (III) or a compound represented by the following formula (IV):

(In formula (IV), n ₃₂ is an integer of 1 to 10,000;
E is a thiol group, an amino group, or _-COOPhNO2 (Ph represents an o-, m-, or p-phenylene group);
R ³¹ represents an alkylene group having 1 to 7 carbon atoms, an alkenylene group having 2 to 7 carbon atoms, -NH-R ³³ -, -CO-R ³³ -, -R ³⁴ -OR ³⁵ -, -R ³⁴ -NH-R ³⁵ -, -R ³⁴ -COO-R ³⁵ -, -R ³⁴ -COO-NH-R ³⁵ -, -R ³⁴ -CO-R ³⁵ -, R ³⁴ -NH-CO-R ³⁵ - or -R ³⁴ -CO-NH-R ³⁵ -, wherein R ³³ represents an alkylene group having 1 to 7 carbon atoms, R ³⁴ represents an alkylene group having 1 to 3 carbon atoms, and R ³⁵ represents an alkylene group having 1 to 5 carbon atoms;
R ³² represents an alkylene group having 1 to 7 carbon atoms, an alkenylene group having 2 to 7 carbon atoms, -NH-R ³⁶ -, -CO-R ³⁶ -, -R ³⁷ -OR ³⁸ -, -R ³⁷ -NH-R ³⁸ -, -R ³⁷ -COO-R ³⁸ -, -R ³⁷ -COO-NH-R ³⁸ -, -R ³⁷ -CO-R ³⁸ -, R ³⁷ -NH-CO-R ³⁸ - or -R ³⁷ -CO-NH-R ³⁸ -, where R ³⁶ represents an alkylene group having 1 to 7 carbon atoms, R ³⁷ represents an alkylene group having 1 to 3 carbon atoms, and R ³⁸ represents an alkylene group having 1 to 5 carbon atoms.

In formula (III), n ₂₁ to n ₂₄ may be the same or different. The closer the values of n ₂₁ to n ₂₄ are, the more uniform the three-dimensional structure can be, and the higher the strength. Therefore, in order to obtain a gel with high strength, it is preferable that they are the same. If the values of n ₂₁ to n ₂₄ are too high, the strength of the gel will be weak, and if the values of n ₂₁ to n ₂₄ are too low, the gel will not easily form due to steric hindrance of the compound. Therefore, n ₂₁ to n ₂₄ are preferably 25 to 250, more preferably 35 to 180, and even more preferably 50 to 120. The molecular weight of the compound represented by formula (III) is preferably 5×10 ³ to 5×10 ⁴ Da, more preferably 7.5×10 ³ to 3×10 ⁴ Da, and more preferably 1×10 ⁴ to 2×10 ⁴ Da.

In the above formula (III), R ²¹ to R ²⁴ are linker moieties connecting the functional group and the core part. R ²¹ to R ²⁴ may be the same or different, but are preferably the same in order to produce a high-strength gel having a uniform three-dimensional structure.

In the above formula (IV), n ₃₂ can be, for example, an integer value of 1 to 10,000, preferably 25 to 5,000, more preferably 50 to 2,500, and even more preferably 100 to 1,000. The molecular weight of the compound represented by formula (IV) is preferably 5×10 ³ to 5×10 ⁴ Da, preferably 7.5×10 ³ to 3×10 ⁴ Da, and more preferably 1×10 ⁴ to 2×10 ⁴ Da.

Here, with respect to each of R ²¹ to R ²⁴ , R ³¹ , and R ³² , examples of the alkylene group having 1 to 7 carbon atoms include a methylene group, an ethylene group, a propylene group, and a butylene group. The alkenylene group having 2 to 7 carbon atoms is a linear or branched alkenylene group having 2 to 7 carbon atoms and having one or more double bonds in the chain, and examples thereof include a divalent group having a double bond formed by removing 2 to 5 hydrogen atoms from adjacent carbon atoms of the alkylene group having 1 to 7 carbon atoms.

In another preferred embodiment, the first polymer is a compound represented by the following formula (I) or a compound represented by the following formula (II):

(In formula (I), n ₁ to n ₄ may be the same or different, and n ₁ to n ₄ are integers of 5 to 300.
A is a maleimidyl group, an N-hydroxy-succinimidyl group, a sulfosuccinimidyl group, a phthalimidyl group, an imidazoyl group, an acryloyl group or a nitrophenyl group;
R ¹ to R ⁴ may be the same or different and each represents an alkylene group having 1 to 7 carbon atoms, an alkenylene group having 2 to 7 carbon atoms, -NH-R ⁵ -, -CO-R ⁵ -, -R ⁶ -OR ⁷ -, -R ⁶ -NH-R ⁷ -, -R ^6- COO-R ⁷ -, -R ⁶ -COO-NH-R ⁷ -, -R ⁶ -CO-R ⁷ -, -R ⁶ -NH-CO-R ⁷ -, or -R ⁶ -CO-NH-R ⁷ -, where R ² represents an alkylene group having 1 to 7 carbon atoms, R ⁶ represents an alkylene group having 1 to 3 carbon atoms, and R ⁷ represents an alkylene group having 1 to 5 carbon atoms.

(In formula (II), n ₁₂ is an integer of 1 to 10,000;
B is a maleimidyl group, an N-hydroxy-succinimidyl group, a sulfosuccinimidyl group, a phthalimidyl group, an imidazoyl group, an acryloyl group or a nitrophenyl group;
R ¹¹ represents an alkylene group having 1 to 7 carbon atoms, an alkenylene group having 2 to 7 carbon atoms, -NH-R ¹³ -, -CO-R ¹³ -, -R ¹⁴ -OR ¹⁵ -, -R ¹⁴ -NH-R ¹⁵ -, -R ¹⁴ -COO-R ¹⁵ -, -R ¹⁴ -COO-NH-R ¹⁵ -, -R ¹⁴ -CO-R ¹⁵ -, R ¹⁴ -NH-CO-R ¹⁵ - or -R ¹⁴ -CO-NH-R ¹⁵ -, wherein R ¹³ represents an alkylene group having 1 to 7 carbon atoms, R ¹⁴ represents an alkylene group having 1 to 3 carbon atoms, and R ¹⁵ represents an alkylene group having 1 to 5 carbon atoms;
R ¹² represents an alkylene group having 1 to 7 carbon atoms, an alkenylene group having 2 to 7 carbon atoms, -NH-R ¹⁶ -, -CO-R ¹⁶ -, -R ¹⁷ -OR ¹⁸ -, -R ¹⁷ -NH-R ¹⁸ -, -R ¹⁷ -COO-R ¹⁸ -, -R ¹⁷ -COO-NH-R ¹⁸ -, -R ¹⁷ -CO-R ¹⁸ -, R ¹⁷ -NH-CO-R ¹⁸ - or -R ¹⁷ -CO-NH-R ¹⁸ -, where R ¹⁶ represents an alkylene group having 1 to 7 carbon atoms, R ¹⁷ represents an alkylene group having 1 to 3 carbon atoms, and R ¹⁸ represents an alkylene group having 1 to 5 carbon atoms.
The second polymer is a compound represented by the following formula (III) or a compound represented by the following formula (IV):

(In formula (IV), n ₃₂ is an integer of 1 to 10,000;
E is a thiol group, an amino group, or _-COOPhNO2 (Ph represents an o-, m-, or p-phenylene group);
R ³¹ represents an alkylene group having 1 to 7 carbon atoms, an alkenylene group having 2 to 7 carbon atoms, -NH-R ³³ -, -CO-R ³³ -, -R ³⁴ -OR ³⁵ -, -R ³⁴ -NH-R ³⁵ -, -R ³⁴ -COO-R ³⁵ -, -R ³⁴ -COO-NH-R ³⁵ -, -R ³⁴ -CO-R ³⁵ -, R ³⁴ -NH-CO-R ³⁵ - or -R ³⁴ -CO-NH-R ³⁵ -, wherein R ³³ represents an alkylene group having 1 to 7 carbon atoms, R ³⁴ represents an alkylene group having 1 to 3 carbon atoms, and R ³⁵ represents an alkylene group having 1 to 5 carbon atoms;
R ³² represents an alkylene group having 1 to 7 carbon atoms, an alkenylene group having 2 to 7 carbon atoms, -NH-R ³⁶ -, -CO-R ³⁶ -, -R ³⁷ -OR ³⁸ -, -R ³⁷ -NH-R ³⁸ -, -R ³⁷ -COO-R ³⁸ -, -R ³⁷ -COO-NH-R ³⁸ -, -R ³⁷ -CO-R ³⁸ -, R ³⁷ -NH-CO-R ³⁸ - or -R ³⁷ -CO-NH-R ³⁸ -, where R ³⁶ represents an alkylene group having 1 to 7 carbon atoms, R ³⁷ represents an alkylene group having 1 to 3 carbon atoms, and R ³⁸ represents an alkylene group having 1 to 5 carbon atoms.
And, one or both of the first polymer and the second polymer are 4-branched polymers.

In other words, the first polymer is a compound represented by formula (I) and the second polymer is a compound represented by formula (III), or the first polymer is a compound represented by formula (I) and the second polymer is a compound represented by formula (IV), or the first polymer is a compound represented by formula (II) and the second polymer is a compound represented by formula (III).

In a preferred embodiment, the gel composition includes a hydrogel formed by gelling a polymer network formed by crosslinking a first polymer and a second polymer. In this specification, "gel" generally refers to a dispersion system of polymers that has high viscosity and has lost fluidity, and has a state in which the storage modulus G' and loss modulus G" have a relationship G' ≧ G". A hydrogel refers to a state in which the gel contains a solvent such as water inside.

The type of solvent that constitutes the gel composition is not particularly limited, but preferably the solvent includes at least one selected from the group consisting of water, organic solvents, and ionic liquids.

Organic solvents include alcohols such as methanol and ethanol, DMSO, etc. Preferably, the solvent is water.

Ionic liquids are formed by the bonding of cations and anions, and are liquid at room temperature (20°C) or at temperatures around that temperature. Any of a variety of known ionic liquids can be used as the ionic liquid.

The cation may be one or more selected from the group consisting of imidazolium ion, pyridinium ion, tetraalkylammonium ion, pyrrolidinium ion, piperidinium ion, tetraalkylphosphonium ion, pyrazolium ion, trialkylsulfonium ion, morpholium ion, guanidinium ion and lithium ion, which may have a substituent. Alternatively, the cation may be a complex cation consisting of a lithium ion and a glyme represented by RO(CH ₂ CH ₂ O) _n -R (R is an alkyl group having 1 to 4 carbon atoms and n is 2 to 6). The lithium ion and the glyme usually form a complex cation in a molar ratio of 1:1.

Examples of anions include halide ions (fluorine, chlorine, iodine, and bromine), tetrafluoroborate ion (BF ₄ ^- ), BF ₃ CF ₃ - , BF ₃ C ₂ F ₅ - , BF ₃ C ₃ F ₇ - , BF ₃ C ₄ F ₉ - , hexafluorophosphate ion (PF ₆ ^- ), bis(fluorosulfonyl)amide ion (FSA ^- ), bis(trifluoromethanesulfonyl)imide ion ((CF ₃ SO ₂ ) ₂ N-, also known as NTf ₂ - and TFSA ^- ), bis(fluoromethanesulfonyl)imide ion ((FSO ₂ ) ₂ N- ), bis(pentafluoroethanesulfonyl)imide ion ((CF ₃ CF ₂ SO ₂ ) ₂ N- ), perchlorate ion (ClO ₄ - ), and tris(trifluoromethanesulfonyl)carbonate ion (CF ₃ SO ₂ ) ₃ C-), trifluoromethanesulfonate (CF ₃ SO ₃ -), dicyanamide ((CN) ₂ N-), thiocyanate (SCN ^- ), nitrate (NO ₃ ^- ), sulfate (SO ₄ ^2- ), thiosulfate (S ₂ O ₃ ^2- ), carbonate (CO ₃ ^2- ), bicarbonate (HCO ₃ ^- ), phosphate, phosphite, hypophosphite, halogen oxide acetates (XO ₄ ^- , XO ₃ ^- , XO ₂ ^- or XO ^- , where X is fluorine, chlorine, bromine or iodine), halogen acetates ((CX _n H _3-n )COO ^- , where X is fluorine, chlorine, bromine or iodine and n is 1 to 3), tetraphenylborate (BPh ₄ ^- ) and its derivatives (B(Aryl) ₄ ^- ) wherein Aryl=a phenyl group having a substituent.

Of _these , preferred ionic liquids include _, for _example , those _having a _cation such as 1-ethyl-3-methylimidazolium ion, [N( _CH3 )(CH3)( _C2H5 ₎ ( _{C2H4OC2H4OCH3} )]+, [N( _CH3 )( _C2H5 )( _C2H5 ) ₍ _C2H4OCH3 )]+ and an anion such as a halogen ion, _a _{tetrafluoroborate} ion, or a bis(trifluoromethanesulfonyl)imide ion ₍ ( _CF3SO2 ) _2N- ⁾ , with an ionic liquid consisting of a 1-ethyl- ₃ -methylimidazolium ion and a bis(trifluoromethanesulfonyl)imide ion (( _CF3SO2 ₎ _2N- ⁾ being particularly preferred.

Preferred ionic liquids that are in a liquid state at room temperature (20°C) or at temperatures around that temperature include those that consist of one or more cations (preferably imidazolium ions or quaternary ammonium ions) represented by the following general formulas (V) to (VIII) and an anion (X-).

In formulas (V) to (VIII), R represents a linear or branched alkyl group having 1 to 12 carbon atoms or a linear or branched alkyl group containing an ether bond and having a total of 3 to 12 carbon atoms and oxygen atoms.

In formula (V), R ¹ represents a hydrogen atom or a linear or branched alkyl group having 1 to 4 carbon atoms. In formula (V), it is preferable that R and R ¹ are not the same.

In formulas (VII) and (VIII), x is an integer from 1 to 4. In formulas (VIII) and (VIII), the two R groups may be joined together to form an aliphatic saturated cyclic group having 3 to 8 members, preferably a 5- or 6-membered ring.

Examples of linear or branched alkyl groups having 1 to 12 carbon atoms include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, t-butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, and dodecyl. The number of carbon atoms is preferably 1 to 8, and more preferably 1 to 6.

Straight-chain or branched alkyl groups having 1 to 4 carbon atoms include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, and t-butyl.

Examples _of straight-chain or branched alkyl groups containing an ether bond and having a total of 3 to ₁₂ carbons and oxygens include _CH2OCH3 , _{CH2CH2OCH3, CH2OCH2CH3, CH2CH2OCH2CH3, (CH2)p(OCH2CH2} ₎ _qOR2 ₍ _wherein _p _is _an _integer _from ₁ to ₄ , q is an integer from 1 to ₄ , ^and _R2 represents _CH3 _or _C2H5 ₎ .

Examples of anions (X ^- ) include tetrafluoroborate ion (BF ₄ ^- ), BF ₃ CF ₃ ^- , BF ₃ C ₂ F ₅ ^- , BF ₃ C ₃ F ₇ ^- , BF ₃ C ₄ F ₉ ^- , hexafluorophosphate ion (PF ₆ ^- ), bis(fluorosulfonyl)amide ion (FSA ^- ), bis(trifluoromethanesulfonyl)imide ion ((CF ₃ SO ₂ ) ₂ N-, NTf ₂ - , TFSA ^- ), bis(fluoromethanesulfonyl)imide ion ((FSO ₂ ) ₂ N ^- ), bis(pentafluoroethanesulfonyl)imide ion ((CF ₃ CF ₂ SO ₂ ) ₂ N ^- ), perchlorate ion (ClO ₄ ^- ), and tris(trifluoromethanesulfonyl)carbonate ion (CF ₃ SO ₂ ) ₃ C ^- ), trifluoromethanesulfonate ion (CF ₃ SO ₃ ⁻ ), dicyanamide ion ((CN) ₂ N ⁻ ), trifluoroacetate ion (CF ₃ COO ⁻ ), organic carboxylate ions and halogen ions can be mentioned as examples.

When the gel composition contains a solvent, the amount of the solvent in the gel composition is not particularly limited, but from the viewpoint of the extensibility and high toughness of the gel composition, it is more preferable that the amount of the solvent is 1% by mass or more and 80% by mass or less, and more preferably 1% by mass or more and 60% by mass or less. In a preferred embodiment, the solvent is an ionic liquid. In another preferred embodiment, the solvent is water.

In the gel composition, the mass ratio of the polymer network to the solvent (polymer network:solvent) is not particularly limited, but is preferably 99:1 to 20:80 in terms of the extensibility and high toughness of the gel composition, and more preferably 99:1 to 40:60 in terms of high toughness. In a preferred embodiment, the solvent is an ionic liquid. In another preferred embodiment, the solvent is water.

If the gel composition contains water, the amount of water in the gel composition may be adjusted by drying the gel composition after production.

The gel composition may further include optional additives.

In the gel composition of the embodiment of the present invention, stretch-induced crystallization occurs due to uniaxial stretching, and crystals of the polyethylene glycol skeleton are formed. Such a gel composition is tougher than a gel composition in which stretch-induced crystallization does not occur.

In a preferred embodiment, the gel composition has a breaking stress of 1 MPa or more, measured by deforming a gel sheet of the gel composition, which has been punched into a dumbbell shape having a thickness of 1 mm in accordance with JIS K 6251 No. 3, by uniaxial stretching at an elongation rate of 12.5%/sec.

According to another aspect of the present invention, there is also provided a gel sheet, which is a sheet formed from the gel composition of any of the above embodiments.

　An overview of the gel composition of an embodiment of the present invention is shown in Figure 1. An embodiment of the gel composition of the present invention is a gel material consisting of a 4-branched polymer network 1 obtained by mixing and reacting a 4-branched polymer having reactive group A at the end and a 4-branched polymer or a linear polymer having reactive group B in a solvent or a solution containing the solvent 2 so that the molar ratio of the ends is the same. At this time, reactive group A and reactive group B are electrophilic functional groups and nucleophilic functional groups (in no particular order), and examples of combinations that are applicable include active ester ends and amino ends, maleimide ends and thiol ends, and maleimide ends and amine ends (the order of the front and back functional groups is no particular). PEG is used as the polymer. The solvent may be a polar solvent or an organic solvent, and examples of polar solvents include water, ionic liquids, and salts. When the PEG concentration in the gel is 40 wt% or more, the PEG molecular weight between crosslinks is 10,000 g/mol or more, and the gel composition is stretched uniaxially, for example as shown by the arrow in Figure 1, stretch-induced crystallization occurs, resulting in a large breaking stress of 1 MPa or more. In this specification, the molecular weight between crosslinks is calculated as the sum of the molecular weight of the arm of the 4-branched polymer having reactive group A and the molecular weight of the arm of the 4-branched polymer having reactive group B, and the molecular weight of each 4-branched polymer arm is measured by gel permeation chromatography (GPC) or matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOFMS).

A typical example of a method for producing a gel composition according to an embodiment of the present invention is described below. However, the present invention is not necessarily limited to this production method.

A method for producing a preferred embodiment of a gel composition includes a step of obtaining a gel composition by crosslinking, in a solvent, a first polymer, which is a four-branched or linear polymer having an electrophilic functional group and a polyethylene glycol backbone, and a second polymer, which is a four-branched or linear polymer having a nucleophilic functional group and a polyethylene glycol backbone. This allows a hydrogel to be obtained in one step.

The initial concentrations of the first polymer and the second polymer are less than the overlap concentration C ^* , and preferably less than 1/3C ^* . Here, the "overlap concentration" (also called "overlap concentration") is the concentration at which polymers in a solvent begin to spatially contact each other, and the overlap concentration C ^* is generally expressed by the following formula:

(wherein _Mw is the weight average molecular weight of the polymer; α is the specific gravity of the solvent; N _A is the Avogadro constant; and R _g is the radius of gyration of the polymer).

The overlap concentration C ^* can be calculated, for example, by referring to Polymer Physics (written by M. Rubinstein and R. Colby). Specifically, for example, the overlap concentration C* can be calculated by measuring the viscosity of a dilute solution using the Flory-Fox equation.

In addition, since this manufacturing method involves gelation in one step, the initial concentration of the 4-branched polymer must be equal to or higher than the critical gelation concentration. Here, "critical gelation concentration" refers to the minimum concentration of raw polymers required to achieve gelation in a system in which a three-dimensional gel is constructed by crosslinking raw polymers, and is also called the minimum gelation concentration. In the present invention, the term critical gelation concentration includes, for example, in a system in which two or more raw polymers are used, not only a case in which the total concentration of the polymers does not reach a concentration that leads to gelation, but also a case in which only one raw polymer has a low concentration, i.e., a case in which gelation does not occur due to the ratio of the raw polymers being unequal.

Generally, the critical gelation concentration (minimum gelation concentration) depends on the type of raw polymer used, but such concentrations are either known in the art or can be easily determined experimentally by those skilled in the art. Typically, it is 0.5 to 5% by weight, with the lower limit being about 1/5 of the overlap concentration.

The crosslinking reaction of the first and second polymers can be carried out by mixing a solvent containing the first and second polymers or a solution containing the solvent. The addition speed, mixing speed, and mixing ratio of each solution are not particularly limited, and can be adjusted appropriately by a person skilled in the art.

As a means for mixing the first and second polymer solutions, for example, a two-liquid mixing syringe such as that disclosed in International Publication WO2007/083522 can be used. The temperature of the two liquids when mixing is not particularly limited, and it is sufficient that the first and second polymers are dissolved and each liquid has fluidity. For example, the temperature of the solutions when mixing can be in the range of 1°C to 100°C. The temperatures of the two liquids may be different, but it is preferable that the temperatures are the same since this makes it easier for the two liquids to mix.

Preferably, in the first embodiment of the method for producing a gel composition of the present invention, the final gel composition can be obtained in a reaction time of 2 hours or less, preferably in a reaction time of 1 hour or less.

Industrial Use of the Invention Compared with previously reported sliding-ring gels that show stretch-induced crystallization, 4-branched polymer gels can be obtained simply by mixing macromers, making them easier to synthesize. Although stretch-induced crystallization has been reported in 3-branched polymer gels, 4-branched cross-linked polymer gels are more common, which is advantageous in expanding the range of applications as a material and reducing manufacturing costs. As shown above, in 4-branched gels, a network can be obtained even by combining 4-branched macromers with linear polymers, so the proportion of inexpensive linear polymers can be increased, and the manufacturing cost can be brought closer to that of general gels.

　Another important point about this invention is that it has discovered stretch-induced crystallization using a wide range of polar solvents, including water and ionic liquids. Hydrogels using water as a solvent are expected to be used as biomedical materials, while ionic gels using salts or ionic liquids are expected to be used as electrochemical materials such as wearable devices and soft actuators. These applications require durability against repeated deformation, and this gel can achieve high mechanical reliability by being toughened by stretch-induced crystallization.

1. Preparation of 4-branched hydrogels Polymer networks were obtained by reacting the ends of 4-branched PEG (TetraPEG) with each other or TetraPEG with linear PEG (LinearPEG) in water. TetraPEG with succinimide as the reactive end and TetraPEG or LinearPEG with amine as the reactive end were dissolved in a buffer. A succinimide-terminated PEG solution and an amine-terminated PEG solution were prepared, and the gelation reaction was initiated by mixing the two solutions. The ends were mixed in an equimolar ratio. McIlvaine buffer solution was used as the buffer. A citric acid solution (0.2 M) and a disodium hydrogen phosphate solution (0.4 M) of the specified concentration were prepared. These were mixed according to McIlvaine TC (1921). “A buffer solution for colorimetric comparison”. J. Biol. Chem. 49 (1): 183-186. to obtain a buffer of the desired pH. Finally, the buffer concentration was adjusted by diluting with pure water. As shown in Table 1, an appropriate buffer was used according to the molecular weight and concentration of the polymer used, and the reaction was allowed to proceed so that the gelation time (the time it takes for the two liquids to lose fluidity after mixing) was approximately 10 minutes.
*Possible combinations of reactive ends include (amine and maleimide, thiol and maleimide, succinimide and amine, etc.)

For example, the gel sheet of Example 1 was manufactured as follows. Amine-terminated Linear PEG (0.8 g) with a molecular weight of 20k was dissolved in a buffer (0.720 mL) with a pH of 3.64. Furthermore, succinimide-terminated Tetra PEG (0.4 g) with a molecular weight of 20k was dissolved in a buffer (1.080 mL) with a pH of 3.64. At this time, the proportion of PEG was 40 wt% of the total. After mixing the two liquids, the mixture was placed in a mold made of Teflon (registered trademark) and left to stand for about a day to create a gel sheet with a thickness of 1 mm. The gel thus produced was punched out into a dumbbell shape (JIS K 6251 No. 3) to prepare a tensile test specimen. The gel sheets of Examples 2-3 and Comparative Examples 1-7 were also manufactured in the same manner, and tensile test specimens were prepared from each gel sheet.

2. Preparation of 4-branched ion gel The gel sheet (Linear 20k + Tetra 20k) of the hydrogel of Example 1 prepared above was immersed in pure water for one day while changing the water several times, and the buffer was replaced with pure water. The sheet was dried at 100 degrees under reduced pressure to remove moisture. An ionic liquid (1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [ _C2mim ][ _NTf2 ]) was added to the sheet from which moisture had been removed so that the solid weight fraction was 40 wt%, and the sheet was left at 100 degrees under reduced pressure for one day. It was confirmed by visual inspection and weight measurement that all the ionic liquid had been absorbed into the sheet, and it was confirmed that the final polymer ratio was 40 wt%. The prepared gel was punched into a dumbbell shape (JIS K 6251 No. 3) to prepare a tensile test specimen.

We also performed an experiment to directly prepare an ion gel without going through a four-arm hydrogel. Using TetraPEG, which has maleimide as the reactive end, and either TetraPEG or linear PEG, which has amine as the reactive end, we prepared a gel in a mixed solvent of N,N-dimethylformamide (DMF) and an ionic liquid (1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [C ₂ mim][NTf ₂ ]), and then removed the DMF by heating and reducing the pressure to obtain an ion gel.

Similarly, ion gels were obtained using the following ionic liquids.
Lithium bis(trifluoromethanesulfonyl)imide (Li[NTf ₂ ])
- Equimolar mixture of lithium bis(trifluoromethanesulfonyl)imide and triglyme ([Li(G3)][NTf ₂ ])
- An equimolar mixture of lithium bis(trifluoromethanesulfonyl)imide and tetraglyme ([Li(G4)][NTf ₂ ]).

These ionic liquids are known as solvated ionic liquids, as described in the following literature: Watanabe M et al. (2018). “From Ionic Liquids to Solvate Ionic Liquids: Challenges and Opportunities for Next Generation Battery Electrolytes”. Bull. Chem. Soc. Jpn. 91 (11): 1660-1682.
For example, the examples are as follows. [C ₂ mim][NTf ₂ ] (1.2 g) and DMF (2.1 mL) are added to amine-terminated LinearPEG (0.8 g) with a molecular weight of 20k, and the mixture is heated to 60 degrees to dissolve. In addition, [C ₂ mim][NTf ₂ ] (0.3 g) and DMF (0.530 mL) are added to maleimide-terminated TetraPEG (0.2 g) with a molecular weight of 10k, and the mixture is heated to 60 degrees to dissolve. After mixing the two liquids, the mixture is placed in a mold made of Teflon (registered trademark) and left to stand at 60 degrees for about a day to create a gel sheet with a thickness of 1 mm. The prepared gel sheet is depressurized at 100 degrees for a day to remove DMF, and the desired ion gel sheet is obtained. At this time, the proportion of PEG is 40 wt% of the total. The prepared gel is punched into a dumbbell shape (JIS K 6251 No. 3) to prepare a tensile test piece. When the types of the two polymers were different from those in this example, they were produced in the same manner.

3. Evaluation of gel sheets
[Measurement/Tensile test]
The measurements were carried out using a precision universal testing machine (Shimadzu Corporation, model number: AG-10kNXPlus). Clamp-type grippers were used. The test specimen was held by tightening a pair of grippers with a screw, and the upper gripper was moved vertically upward to uniaxially stretch the test specimen. The extension speed was 0.125/s, and a stress-strain curve was obtained up to breakage. For the hydrogel, measurements were carried out in a container that encased the entire jig and was filled with liquid paraffin oil to prevent evaporation of water. For the ion gel, measurements were carried out in air.

[Measurement: Wide-angle scattering experiment]
The structural changes during the uniaxial extension test were analyzed using wide-angle X-ray scattering (WAXS). When the sample is irradiated with X-rays, the X-rays are scattered by electrons in the sample. The scattered X-rays interfere with each other, resulting in a scattering image consisting of bright and dark areas. The scattering image reflects the contrast of the electron density in the sample, and has a Fourier transform relationship with the electron density distribution in real space. Therefore, when a peak is observed in the scattering image, it indicates the presence of a repeating structure with a length corresponding to that scattering angle. WAXS targets small structures of about 1-10 Å. Therefore, in this study, a Bragg peak corresponding to PEG crystals is observed.

　Measurements were mainly carried out using NANOPIX (Rigaku, Tokyo, Japan). The wavelength was 1.5 Å, and the camera length was 80 mm. The exposure time for measurements was 5 minutes. The samples used were the same dumbbell-shaped as those used in the uniaxial extension test. The samples were deformed at a strain rate of approximately 0.125/sec, and when the desired degree of extension was reached, the deformation was stopped. X-rays were irradiated while the deformation was stopped to obtain a scattering image. This was repeated while increasing the degree of extension by 100% increments until the sample broke. For hydrogels, measurements were carried out in a container filled with liquid paraffin oil that encased the entire jig to prevent evaporation of water. For ion gels, measurements were carried out in air.

[Experimental results of 4-branched hydrogel]
The stretch-induced crystallization behavior of the gel sheets of the 4-branched hydrogels of Examples 1-3 and Comparative Examples 1-7 is summarized in Table 2. The stress-strain ratio curves when the molecular weight between crosslinking points was changed from 5,000 to 30,000 with the polymer concentration set to 40 w% are shown in Figure 2 (A). In the case of Tetra10k+ Tetra10k (Comparative Example 7) with a molecular weight between crosslinks of 5,000, no stretch-induced crystallization was observed (FIG. 2(B)). However, in the case of Tetra20k+ Tetra20k (Example 3) with a molecular weight between crosslinks of 10,000 (FIG. 2(C)) and Linear 20k+ Tetra 20k (molecular weight between crosslinks of 30,000) (Example 1) in which a linear chain was incorporated into the network in addition to the 4-branch macromer, stretch-induced crystallization was observed (FIG. 2(D)). In the case of Linear 20k+ Tetra 20k and Tetra20k+ Tetra20k in which stretch-induced crystallization occurred, the breaking stress was 1 MPa, which indicates that toughening due to elongation-induced crystallization has occurred (FIG. 2(A)). In addition, elongation-induced crystallization was observed in the gel sheets of Examples 2 and 3, but not in the gel sheets of Comparative Examples 1-6.

The wide-angle X-ray scattering (WAXS) images of the gel sheets in Figures 2(C) and 2(D) in which stretch-induced crystallization was observed are shown in Figures 3 and 4, respectively, showing the scattering profiles in the stretching and perpendicular directions. As a result, it was found that extended chain crystals of PEG chains were observed in the gel sheet (Example 3) manufactured using the polymer Tetra20k + Tetra20k (molecular weight between crosslinks 10,000, PEG concentration 40 wt%), and helical chain crystals of PEG chains were observed in the gel sheet (Example 1) manufactured using the polymer Linear 20k + Tetra20k (molecular weight between crosslinks 30,000, PEG concentration 40 wt%) (indicated by the arrows in Figures 3 and 4, respectively).

Figure 5 (A) shows the stress-elongation ratio curves when the PEG concentration is changed for a gel sheet made of a 4-branched hydrogel with a molecular weight between crosslinks of 30,000, made from Linear20k and Tetra20k. At PEG concentrations of 20 wt% or less (Comparative Examples 1 and 2), no elongation-induced crystallization was observed (Figures 5 (B) and (C)), whereas, as mentioned above, at a PEG concentration of 40 wt% (Example 1), elongation-induced crystallization was observed (Figure 5 (D)).

[Experimental results of 4-branched ion gel]
As shown in Fig. 6(A), a gel sheet of the 4-branched hydrogel of Example 1 (Linear 20k + Tetra 20k, molecular weight between crosslinks 30,000, PEG concentration 40wt%) was dried and then re-swollen with an ionic liquid (Fig. 6(B)). Stretch-induced crystallization was also observed in the tensile test specimen of the 4-branched ionic gel (PEG concentration 40wt%) produced according to the description in "2. Preparation of 4-branched gel". The formation of PEG helical chain crystals was confirmed at an elongation ratio λ = 8 (indicated by the arrow in Fig. 6(C)).

As shown in Figure 7, stretch-induced crystallization was also observed in tensile specimens of four-branched ion gels when the ion gels were prepared directly using the ionic liquids [C ₂ mim][NTf ₂ ], Li[NTf ₂ ], [Li(G3)][NTf ₂ ], and [Li(G4)][NTf ₂ ] without going through the four-branched hydrogel.

In the four-branched ion gel prepared with [C ₂ mim][NTf ₂ ], the formation of PEG crystals and helical crystals was confirmed at an elongation ratio of λ = 9, in the four-branched ion gel prepared with Li[NTf ₂ ], the formation of PEG extended crystals and helical crystals was confirmed at an elongation ratio of λ = 14, and in the four-branched ion gels reswollen with [Li(G3)][NTf ₂ ] and [Li(G4)][NTf ₂ ], the formation of PEG extended crystals was confirmed at an elongation ratio of λ = 14.

Claims

1. A gel composition comprising:
A polymer network formed by crosslinking a first polymer, which is a 4-branched or linear polymer having an electrophilic functional group and a polyethylene glycol backbone, with a second polymer, which is a 4-branched or linear polymer having a nucleophilic functional group and a polyethylene glycol backbone, wherein one or both of the first polymer and the second polymer are 4-branched polymers;
A solvent,
A gel composition, wherein the total amount of the first polymer and the second polymer relative to the amount of the gel composition is 30% by weight or more, and the molecular weight between crosslinking points of the polymer network is 10,000 or more.
The gel composition according to claim 1, wherein the electrophilic functional group of the first polymer is an active ester group, a maleimidyl group, a carboxyl group, an N-hydroxy-succinimidyl group, a sulfosuccinimidyl group, a phthalimidyl group, or a nitrophenyl group.
2. The gel composition of claim 1, wherein the nucleophilic functional group of the second polymer is an amino group, a thiol group, or -CO2PhNO2 , where Ph is an o-, m-, or p-phenylene group.
The gel composition according to claim 1, wherein the solvent comprises at least one selected from the group consisting of water, an organic solvent, and an ionic liquid.
The gel composition according to claim 1, in which crystals of the polyethylene glycol skeleton are formed by uniaxial stretching.
The gel composition according to claim 1, wherein a gel sheet made of the gel composition according to claim 1 punched into a dumbbell shape according to JIS K 6251 No. 3 with a thickness of 1 mm is deformed by uniaxial stretching at an elongation rate of 12.5%/sec, and the breaking stress measured is 1 MPa or more.
The first polymer is a compound represented by the following formula (I) or a compound represented by the following formula (II):

(In formula (I), n 1 to n 4 may be the same or different, and n 1 to n 4 are integers of 5 to 300.
A is a maleimidyl group, an N-hydroxy-succinimidyl group, a sulfosuccinimidyl group, a phthalimidyl group, an imidazoyl group, an acryloyl group or a nitrophenyl group;
R 1 to R 4 may be the same or different and each represents an alkylene group having 1 to 7 carbon atoms, an alkenylene group having 2 to 7 carbon atoms, -NH-R 5 -, -CO-R 5 -, -R 6 -OR 7 -, -R 6 -NH-R 7 -, -R 6- COO-R 7 -, -R 6 -COO-NH-R 7 -, -R 6 -CO-R 7 -, -R 6 -NH-CO-R 7 -, or -R 6 -CO-NH-R 7 -, where R 5 represents an alkylene group having 1 to 7 carbon atoms, R 6 represents an alkylene group having 1 to 3 carbon atoms, and R 7 represents an alkylene group having 1 to 5 carbon atoms.

(In formula (II), n 12 is an integer of 1 to 10,000;
B is a maleimidyl group, an N-hydroxy-succinimidyl group, a sulfosuccinimidyl group, a phthalimidyl group, an imidazoyl group, an acryloyl group, or a nitrophenyl group;
R 11 represents an alkylene group having 1 to 7 carbon atoms, an alkenylene group having 2 to 7 carbon atoms, -NH-R 13 -, -CO-R 13 -, -R 14 -OR 15 -, -R 14 -NH-R 15 -, -R 14 -COO-R 15 -, -R 14 -COO-NH-R 15 -, -R 14 -CO-R 15 -, R 14 -NH-CO-R 15 - or -R 14 -CO-NH-R 15 -, wherein R 13 represents an alkylene group having 1 to 7 carbon atoms, R 14 represents an alkylene group having 1 to 3 carbon atoms, and R 15 represents an alkylene group having 1 to 5 carbon atoms;
R 12 represents an alkylene group having 1 to 7 carbon atoms, an alkenylene group having 2 to 7 carbon atoms, -NH-R 16 -, -CO-R 16 -, -R 17 -OR 18 -, -R 17 -NH-R 18 -, -R 17 -COO-R 18 -, -R 17 -COO-NH-R 18 -, -R 17 -CO-R 18 -, R 17 -NH-CO-R 18 - or -R 17 -CO-NH-R 18 -, where R 16 represents an alkylene group having 1 to 7 carbon atoms, R 17 represents an alkylene group having 1 to 3 carbon atoms, and R 18 represents an alkylene group having 1 to 5 carbon atoms.
2. The gel composition according to claim 1, wherein the second polymer is a compound represented by the following formula (III) or a compound represented by the following formula (IV):

(In formula (III), n 21 to n 24 may be the same or different, and n 21 to n 24 are integers of 25 to 250.
D is a thiol group, an amino group, or -COOPhNO2 (Ph represents an o-, m-, or p-phenylene group);
R 21 to R 24 may be the same or different and each represents an alkylene group having 1 to 7 carbon atoms, an alkenylene group having 2 to 7 carbon atoms, -NH-R 25 -, -CO-R 25 -, -R 26 -OR 27 -, -R 26 -NH-R 27 -, -R 26 -COO-R 27 -, -R 26 -COO-NH-R 27 -, -R 26 -CO-R 27 -, R 26 -NH-CO-R 27 - or -R 26 -CO-NH-R 27 -, where R 25 represents an alkylene group having 1 to 7 carbon atoms, R 26 represents an alkylene group having 1 to 3 carbon atoms, and R 27 represents an alkylene group having 1 to 5 carbon atoms.

(In formula (IV), n 32 is an integer from 1 to 10,000;
E is a thiol group, an amino group, or -COOPhNO2 (Ph represents an o-, m-, or p-phenylene group);
R 31 represents an alkylene group having 1 to 7 carbon atoms, an alkenylene group having 2 to 7 carbon atoms, -NH-R 33 -, -CO-R 33 -, -R 34 -OR 35 -, -R 34 -NH-R 35 -, -R 34 -COO-R 35 -, -R 34 -COO-NH-R 35 -, -R 34 -CO-R 35 -, R 34 -NH-CO-R 35 - or -R 34 -CO-NH-R 35 -, where R 33 represents an alkylene group having 1 to 7 carbon atoms, R 34 represents an alkylene group having 1 to 3 carbon atoms, and R 35 represents an alkylene group having 1 to 5 carbon atoms;
R 32 represents an alkylene group having 1 to 7 carbon atoms, an alkenylene group having 2 to 7 carbon atoms, -NH-R 36 -, -CO-R 36 -, -R 37 -OR 38 -, -R 37 -NH-R 38 -, -R 37 -COO-R 38 -, -R 37 -COO-NH-R 38 -, -R 37 -CO-R 38 -, R 37 -NH-CO-R 38 - or -R 37 -CO-NH-R 38 -, where R 36 represents an alkylene group having 1 to 7 carbon atoms, R 37 represents an alkylene group having 1 to 3 carbon atoms, and R 38 represents an alkylene group having 1 to 5 carbon atoms.
A gel sheet formed from the gel composition according to any one of claims 1 to 7.